"One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms – why not silicon-aluminium men at once? – wandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace."
- H.G. Wells
It may be possible for life to form out of complex chains of Silicon instead of the chains of Carbon common to Life As We Know It. Such lifeforms would probably live in environments toxic to human life. The most common otherness tropes for a silicon-based lifeform is some sort of crystaline or insectoid life, often with pronounced facets. On classic Star Trek, however, there was a critter that looked more like a giant pile of scrambled eggs or meatballs. Another logical possibility is for fiberglass-like silicon threads, as either webbing or body structure.
Silicon life would probably be based on Silicones, polymers of alternating silicon and oxygen. While in general silicones are less stable than hydrocarbons, they would have an advantage in certain environments. In particular, in an atmosphere or environment where sulfuric acid was common, silicones would have greater resilience than carbon-based molecules.
Silicones do run into some troubles, however. Namely, silicon that gets exposed to oxygen often forms silicon dioxide. Since it would be the analog of carbon dioxide, there's reason to believe such life would exhale it as part of respiration - breathing out dust and sand. Silicon dioxide is a solid at most temperatures, and not water-soluble, so you'd have problems removing this waste from cells, recycling biological material, etc. Now, "life may find a way", and in the case of silicone-life it's possible the solution would be ammonia. See Ammonia as Biological Solvent for a discussion of how it might work.
So, the environments most friendly to Silicon-based life are rich in ammonia and sulfuric acid. Ammonia's boiling point is below water's freezing point, so the planet would either be very cold or have extreme pressure (because at higher pressures, the boiling point of ammonia is much higher) - say, 60 atmospheres. Ice Giants and places like Titan, moon of Saturn might be viable habitats for silicon-based life. In other words, any planet likely to be hospitable to silicon life isn't going to be nice for humans. Maybe, just maybe, you might be able to get around that by having a Cloud Planet scenario - a layered Gas Giant that has some breathable earth-atmosphere at the high altitudes and a high-pressure silicone-ammonia environment further down. The notion of shared environments is iffy, to say the least.
Molten Silicon Life
Alternately, since silicon structures survive great temperatures, it's possible silicon life might exist in places of extreme heat. These thermophile / extremophile lifeforms would be made of molten silicon. This version of silicon life might exist deep within planetary cores. They may even form a shadow biosphere inside our own planet, of which we are currently ignorant. They may draw energy from infraradiated heat (see Non-Green Photosynthesizers) or from the oxidization of iron. Nothing I've read really suggests what would be used as a solvent by such creatures - but I assume it'd probably be some extremely hot metal that's normally a solid on the surface of the earth.
For more ideas, read the H.G. Wells quote at the top of this page.
Obviously, the prospects of humans interacting with molten silicon life are no more promising than our capacity to interact with super-chilled silicone-ammonia life.
Silanes are compounds of hydrogen and silicon, and have much in common with hydrocarbons, so they could be a basis for life. Hydrogen is the most common element in the universe, so Silanes are pretty common, and a ready building block. However, Silanes have a propensity to spontaneously decompose. Rather than being crystaline lifeforms, Silane-based life is more likely to be single-celled organisms, or a cellular colony resembling sludge, broth or scrambled eggs. Life based on Silanes may need to be modular, and hearty enough to survive the degeneration and spontaneous death of it's member cells. As such, it may have a hive mind.
Note that you still have the issues of solvency and silicon dioxide in this life model, but perhaps the single-celled version can respirate sand a little better than a multicellular organism. It also has the temperature restrictions of the previous categories, probably needing to be either extremely cold, extremely hot, or under a lot of pressure.
Here on earth, some carbon-based organisms incorporate bits of Biogenic Silica into their cellular membranes. Diatoms, and some times of holoplankton use it significantly, as do sponges and some plants. In humans, trace amounts of silicon are used in our connective tissues, collagen, bones and teeth. So, it's not impossible that life elsewhere might use both carbon and silicon, and possibly at a more silicon-friendly ratio than most earth-life. You never know what random mutation and natural selection will dredge up.
- It's possible you could have a lifeform that uses carbon and water for it's metabolism, but silica for its structural molecules.
- Or one that goes through various life stages, with a carbon-predominant larval stage and a silicon-derived adult body - or vice-versa.
- Or a symbiotic relationship where a silicon shell organism protected the more vulnerable carbon bacteria inside it, and the bacteria produced food/energy/respiration for the silicon critter that was protecting them from sulferic acid.
Here on Earth there's more Silicon than Carbon. The fact that silicon outnumbers carbon on earth, yet carbon life predominates here, suggests that silicon is not as capable or fitting for sustaining life. In the universe at large, however, there's about 10 times as much carbon as silicon - so even if silicon can become life, it's probably going to be outnumbered by carbon. However, that doesn't mean it can't happen given the correct circumstances.
Many of the problems with silicon life may just be Carbon Chauvinism on our part. We're used to a planet with plentiful water and oxygen, which makes Silicon life seem pretty bad. If oxygen is scarce, gaseous silicon won't turn into sand. As mentioned, Ammonia as a Biological Solvent might fix the problems with dissolving silicon. Sulferic acid mists or oceans could help, as could extremes of temperature or pressure. There's possibilities of other atmospheres making this work as well, such as those with high concentrations of hydrogen. In general, the further you get from earth-like conditions, the more plausible silicon life becomes. The dead of space..? Probably "no" - but there's some reason to believe Silicon life might be well adapted to space exposure and hard vacuum, too.
Silicon has many chemical properties in common with Carbon, and occupies a similar niche on the Periodic Table.
- Silicon is heavier than carbon, but forms less stable bonds. This means silicon life may be dense and bulky, but easily fractured or eroded.
- In earth-like conditions, silicon forms fewer complex molecules than carbon, so silicon life may prove simpler than the more advanced forms of Life As We Know It.
Silicon rarely creates compounds with Chirality (chemistry) - see Alien Amino Acids for a brief discussion of chirality. Nearly all amino acids on earth are "left-handed" but silicon practically can't be. Most silicon compounds are neither left- nor right-handed (being either "ambidextrous" or "handless" depending on your point of view), which may or may not interfere with it's ability to support life. Silicon-based life would need silicone- or silane-based equivalents of amino acids, and nothing like that has been observed yet.
Game and Story Use
- Silicon Dioxide is a solid at earth temperatures. If the silicon-life doesn't live at extreme temperatures, it may take in gases, and expel powder or granules. Thinking about that, since silicon dioxide is a common ingredient in ordinary sand, perhaps silicon-life is a good match for a Desert Planet.
- Molten Silicon Life may exist on Hot Jupiters, or deep with the cores of planets.
- See also shadow biosphere - because molten silicon life could possibly be miles below our feet at this very moment.
- Earth Elementals or Gnomes could be described or defined as silicon-based lifeforms. The nice thing here is that magic allows you to handwave the respiration and solvency issues. They could live at high temperatures, but also be just fine at the earth's surface.
- For a really interesting notion that the first life on Earth may have been somewhat silicon-based, see Clay Theory.
- Silicon-based life would be very alien to carbon-based life. We may not realize it to be life, and it may make similar poor assumptions about us. See Eldritch Abomination, Aberration, and Starfish Aliens for more ideas.
- A silicon-based lifeform would be high interest to a xenobiologist or cryptozoologist.